[FIX] RESTXQ: Default element namespaces
[FIX] XQuery, Error handling
[FIX] `hof:fold-left1(..)` and superfluous `item()*` type checks.
Merge pull request #813 from LeoWoerteler/var-checks

[FIX] `hof:fold-left1(..)` and superfluous `item()*` type checks.
[FIX] GUI, Buttons, #814: focus previous/next component when pressing
      left/right or up/down
Merge branch 'next' of https://github.com/dimitarp/basex into next
[FIX] GUI, Editor: line numbers (immediate rendering)
[FIX] GUI, Editor, #811: CMD+DEL deletes first character from row
[MOD] CSV/JSON conversion: check validity, 'lax' decoding revised. #810
[FIX] Core, File operations: always operate on absolute path.
[ADD] GUI, Editor: content search; text field hints added
[FIX] XQuery, HTTP Module: accept empty results
[ADD] GUI Editor, Project View: open files natively
[MOD] GUI Editor: disallow use of undeclared variables
[MOD] GUI Editor, Project View: sort tree entries (relevant for Linux)
[MOD] GUI Editor, code completion: automatically close comments
[MOD] XQuery, splitter: hide invisible panels
[MOD] GUI, Project View: update tree when files is stored
[MOD] GUI: keyboard shortcuts added
[MOD] GUI Editor: shortcuts added to popup menu